Extension-ready spinal support system with vascular-safe pedicle screw

ABSTRACT

A “vascular-safe” pedicle screw and extension-ready spinal support system. The pedicle screw includes a self-tapping flute that recessed into a threaded shaft to define terminations of the threads at a face of said self-tapping flute. The terminations are curved to define a convex profile that extends from a root to a crest of the threads at said face of said self-tapping flute. The curved terminations tend to push soft tissue aside as opposed to slicing or tearing through the soft tissue, so that the self-tapping flute is less likely to slice into vascular vessels. The distal portion of the pedicle screw may also include depressions that reduce the circumferential contact area of the pedicle screw in the direction of rotation, which increases the applied pressure to the soft tissue for a given applied rotational force. The increased pressure augments penetration of the pedicle screw through tissue without resort to sharp cutting edges.

RELATED APPLICATIONS

This application is a continuation-in-part of International PatentApplication No. PCT/US2018/030928, filed May 3, 2018, and is acontinuation-in-part of U.S. patent application Ser. No. 15/970,368,filed May 3, 2018, which claims the benefit of U.S. Provisional PatentApplication No. 62/500,820, filed on May 3, 2017, and of U.S.Provisional Patent Application No. 62/500,719, filed May 3, 2017, thedisclosures of which are hereby incorporated by reference herein intheir entirety. This application also claims the benefit of U.S.Provisional Patent Application No. 62/867,616, filed Jun. 27, 2019, thedisclosure of which is hereby incorporated by reference herein in itsentirety.

FIELD OF THE DISCLOSURE

The present disclosure relates to spinal support systems and moreparticularly to spinal support systems that are readily extensive withpedicle screws that inhibit cutting or slicing of soft tissues.

BACKGROUND OF THE DISCLOSURE

Implementation of various spinal surgical techniques often requires theuse of spinal support rods that are anchored to the vertebrae throughthe use of pedicle screws to provide stabilization of the spine duringhealing or correction. Examples include maintaining adjacent vertebraestationary so that bone growth tissue can bridge the vertebrae in aspinal fusion process. Another example is the use of spinal support rodsto apply a coercive force to the spine for corrective purposes (e.g.,correction of scoliosis).

The pedicle screws are tapped into the vertebrae of the spine for theanchoring. The anatomy of the human body is such that the spine includesand is proximate to numerous vascular vessels. A concern is that thepedicle screw could contact and damage the vascular vessels during thetapping process.

Also, the spinal rods are typically disposed in rod receptacles at thehead of the pedicle screw. The rod receptacles typically definediametrically opposed slots, with the spinal rod extending through theslots. The formation of the slots further define opposed wall segmentsthat extend proximally on lateral sides of the spinal rod. The spinalrod is typically held within the slots with a set screw that threadablyengages the wall segments. Over time, creep stresses may cause the wallsegments to plastically deform radially outward and away from the setscrew, with an accompanying decrease the clamping force of the set screwagainst the spinal rod due to loss of elasticity of the wall segments.This can cause loosening of the assembly and slippage of the residentspinal rod within the spinal rod receptacle. In some instances, torquerequirements, particularly where the spinal rods are subject to hightension forces, can cause the set screw to slip within the spinal rodreceptacle during implantation.

In some cases, surgery is later required to treat other vertebrae of thesame patient. “Extension” systems have been developed which enableadditional spinal support rods to be coupled to existing spinal rods orpedicle screws of the previous surgery, thereby reducing surgical traumaand recovery times. Such an extension system is found, for example, atU.S. Pat. No. 9,655,654 to Abbasi entitled “Spinal Rod Support Structurewith Clamp,” owned by the owner of the current application, thedisclosure of which is incorporated by reference herein in its entiretyexcept for patent claims and express definitions contained therein.

The present application identifies shortcomings and limitations in theart of spinal rod extension systems and provides improvements to remedysuch shortcomings and limitations.

SUMMARY OF THE DISCLOSURE

Various embodiments of the disclosure are directed to a “vascular-safe”pedicle screw. The vascular-safe pedicle screw includes one or moreself-tapping flutes that interrupt or terminate the threads at severalpoints along the threads. These terminations are configured to push softtissue aside as opposed to slicing or tearing through the soft tissue,so that the self-tapping flute is less likely to slice into vascularvessels, while still effectively tapping into bone tissue. The distalportion of the pedicle screw may also include features that reduce thecircumferential contact area of the pedicle screw in the direction ofrotation, to increase the applied pressure to the soft tissue for agiven applied rotational force for augmenting penetration of the pediclescrew without resort to sharp cutting edges. Some embodiments of thedisclosed pedicle screw are not only vascular-safe, but alsoself-tapping, self-steering, self-centering, or a combination thereof.

Various embodiments of the disclosure also provide an “extension ready”spinal support system that enables the extension to be accomplished withreduced disturbance to an existing spinal support structure relative toconventional extension systems. In some embodiments, existingconnections between the existing spinal support rod and pedicle screwscan remain intact while extension rod receptacles are mounted directlyto the existing base rod receptacles. In some embodiments, the extensionreceptacle may be polyaxial with respect to the base receptacle. In someembodiments, the extension receptacle is provided with a low profile(i.e., shortened axial projection length from the base rod receptacle)relative to the polyaxial embodiment by eliminating polyaxial structurebetween the base receptacle and the extension receptacle and theadditional axial lengths associated therewith and providing a monoaxialstructure that rotates about but does not pitch relative to theextension axis.

In various embodiments of the disclosure, a spinal support system isdisclosed where a unitary reinforcement cap threadably engages both theinternal and the external surfaces of the wall segments of the rodreceptacle. By doubling the number of threads that are engaged,additional strength and structural integrity are realized over standardset screw arrangements. Also, because of the unitary construction,wherein an internal set screw portion and an external skirt portion areintegral to the reinforcement cap, lateral movement between the setscrew portion and the skirt portion is limited.

In some embodiments, the unitary reinforcement cap does not threadablyengage the external surface, but instead slides over the externalsurface to capture and support to the rod receptacle. Accordingly, suchembodiments do not require threads on the external surface of the rodreceptacle, and can be configured for a retrofit of conventional rodreceptacles.

In some embodiments, the threads are of a canted cantileverconstruction, wherein the interfaces of threadably engaged componentsare tailored to generate reaction force vectors that prevent slippage atthe interfaces.

Conventional spinal rod extension systems exist where extension spinalsupport rod receptacles (also referred to as extension “tulips” in theparlance of the spinal support arts) can be mounted “piggyback” to anexisting or “base” spinal support rod receptacle. However, conventionalsystems require that the connection between the base spinal support rodreceptacle be disturbed or modified. For example, in some systems, theset screw securing the base spinal rod to the base receptacle must beremoved so that the extension receptacle can utilize the interiorthreads of the base receptacle. This releases the base spinal supportrod from the base receptacle, thus requiring that the base spinalsupport rod be reset before the surgeon can proceed with implantation ofthe extension system. Often, the required structure that extends fromthe base receptacle is of undesirable axial length, causing concealmentissues.

Another drawback of conventional rod receptacle arrangements is theseparation or “splaying” of the tulip. When a conventional set screw isseated within a conventional tulip, forces are generated on the tulipwall portions that force the tulip wall sections away from each other.This separation of the tulip walls is referred to as “splaying.”

Various embodiments of the disclosure alleviate one or more of theseshortcomings and limitations. The disclosed embodiments enable extensionof the spinal support structure without need for removing the tulips orpedicle screws of the already-implanted spinal support structure.Certain embodiments alleviate the need to remove the set screw from theinterior threads of the base receptacle. As with conventional rodreceptacles, the disclosed base rod receptacle includes interior threadsto which a set screw is coupled for securing the base spinal supportrod. In addition, the disclosed system further includes exterior threadsto which an optional extension assembly can be mounted at a later time.By utilizing the exterior threads for mounting the extension system, theset screw that secures the base spinal support rod to the base rodreceptacle does not have to be removed from the interior threads of thebase rod receptacle. In this way, the base spinal support system canremain intact and undisturbed as the extension system is coupledthereto.

Other embodiments may involve removal of the set screw from the basereceptacle, but utilize a reinforced, dual threaded or capped base thatis stronger and more robust than standard set screw arrangements andprevents splaying of the tulip.

Structurally, the vascular-safe pedicle screw comprises a head portion,a threaded shaft portion distal to the head portion and includingthreads that extend to a distal end portion of the threaded shaftportion, and a self-tapping flute at the distal end portion of thethreaded shaft portion. The self-tapping flute is recessed into thethreaded shaft to define at least one terminated thread at a face of theself-tapping flute. In some embodiments, the terminated threads arecurved to define a convex profile that extends in a tangential directionfrom proximate a root of the at least one terminated thread at the faceof the self-tapping flute to a crest of the at least one terminatedthread. The distal end portion of the threaded shaft portion may alsodefine a depression at a leading end of the thread. In some embodiments,the head portion and the threaded shaft portion define a central passagethat passes therethrough. The threaded portion may define a plurality offenestrations that are in fluid communication with the central passage.In some embodiments, the threads are double threads. The threaded shaftportion may include dulled edges, for example polished or roughenededges. In some embodiments, the threaded shaft portion defines a surfaceroughness having arithmetical mean deviation that is in a range of 5micrometers to 15 micrometers inclusive.

In various embodiments of the disclosure, a spinal rod support system isdisclosed, comprising a pedicle screw, including a head portion, athreaded shaft portion distal to the head portion and including threadsthat extend to a distal end portion of the threaded shaft portion, and aself-tapping flute at the distal end portion of the threaded shaftportion, the self-tapping flute being recessed into the threaded shaftto define terminations of the threads at a face of the self-tappingflute, the terminations being defining a convex profile that extends ina tangential direction from a root to a crest of the threads at the faceof the self-tapping flute. In some embodiments, the spinal rod supportsystem includes a base rod receptacle coupled to the pedicle screw, andan extension rod receptacle configured to couple to the base rodreceptacle. The threaded shaft portion may include dulled edges. In someembodiments, a base portion is configured to threadably couple to thebase rod receptacle, the base portion including a skirt portion thatextends from a mounting platform, the mounting platform defining acenter hole accessible from a proximal face of the mounting platform,the extension rod receptacle extending into the center hole. The baserod receptacle may define exterior threads, with the skirt portion ofthe base defining interior threads configured to mate with the exteriorthreads of the base rod receptacle. In some embodiments, the spinal rodsupport system comprises a reinforcement cap configured to couple to oneof the base rod receptacle and the extension rod receptacle, thereinforcement cap including a set screw portion and a skirt portion,wherein the base rod receptacle defines interior threads, the extensionrod receptacle defines interior threads, and the set screw portion ofthe reinforcement cap defines exterior threads configured to mate witheither of the interior threads of the base rod receptacle or theinterior threads of the extension rod receptacle. In some embodiments,the extension rod receptacle defines exterior threads. The skirt portionof the reinforcement cap may define interior threads configured to matewith either of the exterior threads of the base rod receptacle or theexterior threads of the extension rod receptacle.

In some embodiments, a “reverse canted cantilever” arrangement isdisclosed, wherein the interior threads of the base rod receptacle andthe interior threads of the extension rod receptacle each define a firstcanted cantilever profile that extends radially inward and in a distaldirection, and the exterior threads of the base rod receptacle and theexterior threads of the extension rod receptacle each define a secondcanted cantilever profile that extends radially outward and in aproximal direction.

In various embodiments of the disclosure, a spinal support systemcomprises a first rod receptacle for mounting to a pedicle screw, thefirst rod receptacle having a first side wall that includes a firstinterior surface and a first exterior surface. The first side walldefines a first pair of diametrically opposed slots that extend axiallyalong the first side wall and are open at a proximal end of the firstrod receptacle, the first interior surface defining first interiorthreads. A first set screw includes threads configured to mate with thefirst interior threads of the first rod receptacle. The first exteriorsurface defines first exterior threads. An extension assembly includinga base portion mounted to a second rod receptacle, the base portionincluding a mounting platform and a skirt portion that extends from themounting platform, the skirt portion including interior threads formating with the first exterior threads of the first rod receptacle, thesecond rod receptacle having a second side wall that includes a secondinterior surface and a second exterior surface. The second exteriorsurface of the second rod receptacle may include second exteriorthreads. The second side wall defines a second pair of diametricallyopposed slots that extend axially along the second side wall and areopen at a proximal end of the second rod receptacle, the second interiorsurface defining second interior threads. A second set screw includesthreads configured to mate with the second interior threads of thesecond rod receptacle.

The extension assembly may include a pivot member that attaches thesecond rod receptacle to the base portion, the pivot member including ahead portion and a shaft portion, the shaft portion including shaftthreads formed on an exterior surface thereof, the shaft portiondefining a pivot axis. In some embodiments, the second rod receptacledefines an opening at a distal end thereof, the opening sized toaccommodate passage of the shaft portion of the pivot member. Themounting platform of the base portion may define a center hole forreceiving the pivot member, and may include interior threads for matingwith the external shaft threads. In some embodiments, the center hole isa through hole.

In some embodiments, the second rod receptacle includes an internalflange having an interior face, the head portion of the pivot memberbeing dimensioned to register against the interior face of the internalflange to secure the second rod receptacle to the base portion. The headportion of the pivot member may be one of a flat head and a countersinkhead, and the interior face of the internal flange of the second rodreceptacle may conforms to the head portion to enable selectivemonoaxial rotation about the pivot axis.

In some embodiments, a pedicle screw coupled to a distal end portion ofthe first rod receptacle. The pedicle screw and the first rod receptaclemay be configured for polyaxial rotation of the first rod receptacleabout a head of the pedicle screw. In some embodiments, a lock ringconfigured to engage a spinal support rod, the lock ring including adistal face that conforms to the pedicle screw and a proximal face thatincludes one or more malleable features for engaging the spinal supportrod. The one or more malleable features may be plastically deformable.In some embodiments, the one or more malleable features includes araised ridge. In some embodiments, the raised ridge is annular ring.

Some embodiments include a cap including a skirt portion having interiorthreads for mating with the second exterior threads of the second rodreceptacle. In some embodiments, the skirt portion is not threaded, butinstead slides over the exterior of the base rod receptacle to preventsplaying. An exterior surface of the skirt portion may define aplurality of flats, each of said plurality of flats being parallel tothe pivot axis. In some embodiments, one or more of the first interiorthreads, the first exterior threads, the second interior threads, thesecond exterior threads, and the internal threads of the center holedefine a canted cantilever profile. The canted cantilever profile mayextend radially and variously in a distal direction or a proximaldirection.

In various embodiments of the disclosure, a spinal rod support systemcomprises an extension rod receptacle, a base portion including amounting platform that defines a center hole accessible from a proximalface of the mounting platform, and means for coupling the extension rodreceptacle to the base portion. In some embodiments, the means forcoupling the extension rod receptacle to the base portion includes apivot member threadably engaged with the center hole. In someembodiments, the means for coupling the extension rod receptacle to thebase portion includes a pivot member swaged to the center hole. In someembodiments, the means for coupling the extension rod receptacle to thebase portion includes a pivot member fused to the center hole. The pivotmember may be welded to the center hole at a distal face of the baseportion, for example, a distal face of the mounting platform.

The spinal support system may further comprise a base rod receptacle,and means for coupling the base portion to the base rod receptacle. Insome embodiments, the base portion includes a skirt portion, and themeans for coupling the base portion to the base rod receptacle includesthreaded engagement of the skirt portion to the base rod receptacle. Insome embodiments, the base portion includes a set screw portion that isunitary with and extends from a distal face of the mounting platform,and the means for coupling the base portion to the base rod receptacleincludes threaded engagement of the set screw portion to the base rodreceptacle. In some embodiments, the set screw portion and the baseportion include mating threads that define canted cantilever profilesfor the threaded engagement. The canted cantilever profiles may extendradially and in a distal direction. In some embodiments, the spinalsupport system comprises a base spinal support rod disposed in the baserod receptacle, the set screw portion being configured to clamp the basespinal support rod within the base rod receptacle. In some embodiments,the base portion includes a set screw portion that is unitary with andextends from a distal face of the mounting platform, and the means forcoupling the base portion to the base rod receptacle includes threadedengagement of the set screw portion to the base rod receptacle. In someembodiments, the means for coupling the extension rod receptacle to thebase portion enables only monoaxial rotation of the extension rodreceptacle about a pivot axis.

In various embodiments of the disclosure, a method of fabricating anextension assembly for a spinal support system comprises: inserting apivot member into a rod receptacle so that a shaft portion of the pivotmember extends from a distal end of the rod receptacle; disposing theshaft portion in a center hole of a base portion so that an internalflange of the rod receptacle is captured between a head portion of thepivot member and the base portion so that an axial gap dimension definedbetween the head portion and the base member is greater than an axialthickness of the internal flange; and securing the pivot member to thebase portion. In some embodiments, the step of disposing and the step ofsecuring includes threadably engaging the shaft portion with the centerhole. In some embodiments, the step of disposing includes registering astop on the pivot member against the base portion to define the axialgap dimension. In some embodiments, the step of securing includes one offusing and swaging the shaft member to the base portion.

In various embodiments of the disclosure, a reinforcement cap isdisclosed for a spinal support system, comprising: a platform portionincluding a distal face and a proximal face separated by a perimeterportion; a skirt portion that extends from the distal face of theplatform portion, the skirt portion including an interior surface, atleast a portion of the interior surface including interior threadsformed thereon; and a set screw portion that extends from the distalface of the platform portion, the set screw portion being surrounded bythe skirt portion and including an exterior surface that faces radiallyoutward, at least a portion of the exterior surface including exteriorthreads formed thereon, the set screw portion defining a rotation axis,the set screw portion and the skirt portion being concentric about therotation axis to define an annular gap between the exterior threads ofthe set screw portion and the interior threads of the skirt portion. Theskirt portion may include an exterior surface that is tangential withthe perimeter portion. In some embodiments, the set screw portionextends distally beyond the skirt portion. The skirt portion may includean exterior surface that defines a plurality of flats, each of theplurality of flats being parallel to the rotation axis.

In some embodiments, the exterior threads of the set screw portiondefine a canted cantilever profile. Likewise, in some embodiments, theinterior threads of the skirt portion define a canted cantileverprofile. The canted cantilever profile of the set screw portion mayslope in a distal direction and toward the rotation axis. The cantedcantilever profile of the set screw portion may slope in a distaldirection and away from the rotation axis.

In some embodiments, the platform portion defines a socket accessiblefrom the proximal face, the socket being concentric with and extendingalong the rotation axis. The platform portion may also define a tappedcenter hole accessible from the proximal face, the tapped center holebeing concentric with and extending along the rotation axis, the socketextending distally from the tapped center hole. The reinforcement capmay further define a center passage concentric with the rotation axisand extending from the socket through a distal end of the set screwportion.

In various embodiments of the disclosure, a spinal support systemcomprises the reinforcement cap as described above; and a spinal rodreceptacle having a side wall that includes an interior surface and anexterior surface, the side wall defining a pair of diametrically opposedslots that extend axially along the side wall and are open at a proximalend of the spinal rod receptacle, the interior surface of the spinal rodreceptacle defining interior threads, the exterior surface of the spinalrod receptacle defining first exterior threads, wherein the interiorthreads of the spinal rod receptacle are configured to mate with theexterior threads of the set screw portion of the reinforcement cap, andthe exterior threads of the spinal rod receptacle are configured to matewith the interior threads of the skirt portion of the reinforcement cap.In various embodiments, a spinal rod is configured for insertion intothe diametrically opposed slots. A pedicle screw may be disposed withinthe spinal rod receptacle, a shaft of the pedicle screw extendingdistally from the spinal rod receptacle.

In various embodiments of the disclosure, a method for securing a spinalsupport rod to a spinal rod receptacle is disclosed, comprising: (a)disposing a spinal support rod through diametrically opposed slots of aspinal rod receptacle; (b) threadably engaging interior threads of askirt portion of a reinforcement cap with exterior threads of the spinalrod receptacle; (c) simultaneously with step (b), threadably engagingexterior threads of a set screw portion of the reinforcement cap withinterior threads of the spinal rod receptacle; and (d) tightening thereinforcement cap against the spinal support rod. In some embodiments,step (d) includes driving the reinforcement cap with a tool that mateswith a socket formed on the reinforcement cap. In some embodiments, step(d) includes driving the reinforcement cap with a tool that engagesflats formed on the skirt portion of the reinforcement cap.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an extension ready base assemblyaccording to an embodiment of the disclosure;

FIG. 1A is an upper perspective view of a base rod receptacle of FIG. 1in isolation according to an embodiment of the disclosure;

FIG. 2 is a sectional view of an extensible spinal support system infull assembly with the extension ready base assembly of FIG. 1 accordingan embodiment of the disclosure;

FIG. 3 is an enlarged, partial sectional view of a lock ring in theassembly of FIG. 2 according to an embodiment of the disclosure;

FIG. 4 is an enlarged, partial sectional view of a threaded wall portionof a base rod receptacle of the assembly of FIG. 2 according to anembodiment of the disclosure;

FIG. 5 is an enlarged, partial sectional view of a threaded wall portionof an extension rod receptacle of the assembly of FIG. 2 according to anembodiment of the disclosure;

FIGS. 6 through 8 are sectional views of alternative extensible spinalsupport systems in full assembly with the extension ready base assemblyof FIG. 1 according embodiments of the disclosure;

FIG. 9 is a perspective view of a platform of an extensible spinalsupport system with exterior wrench flats according to an embodiment ofthe disclosure;

FIGS. 10 and 11 are sectional views depicting assembly of an extensionsub-assembly having a threaded pivot member according to an embodimentof the disclosure;

FIGS. 12 and 13 are sectional views depicting assembly of an extensionsub-assembly having a welded pivot member according to an embodiment ofthe disclosure;

FIGS. 14 and 15 are sectional views depicting assembly of an extensionsub-assembly having a riveted pivot member according to an embodiment ofthe disclosure;

FIGS. 16 and 17 are sectional views of extension sub-assemblies withbase portions that have integral set screw portions according to anembodiment of the disclosure;

FIG. 18 is a perspective view of the pedicle screw of FIGS. 1 and 2according to an embodiment of the disclosure;

FIG. 18A is a top axial view of the pedicle screw of FIG. 18 accordingto an embodiment of the disclosure;

FIG. 18B is a sectional view of the pedicle screw at plane B-B of FIG.18 according to an embodiment of the disclosure;

FIG. 19 is a perspective view of an alternative pedicle screw accordingto an embodiment of the disclosure;

FIG. 20A is an enlarged, distal end view of the pedicle screw of FIG. 19according to an embodiment of the disclosure;

FIGS. 20B and 20C are an enlarged, partial perspective views of thepedicle screw of FIG. 19 according to an embodiment of the disclosure;

FIG. 21 is a top axial view of a set screw according to an embodiment ofthe disclosure;

FIG. 22 is a section view of the set screw XXII-XXII of FIG. 21 at planeaccording to an embodiment of the disclosure;

FIG. 23 is a perspective view of the set screw of FIG. 21 according toan embodiment of the disclosure;

FIG. 24 is a perspective view of a lock ring according to an embodimentof the disclosure;

FIG. 25 is an upper perspective view of a spinal support systemaccording to an embodiment of the disclosure;

FIG. 26 is a partial sectional view of the spinal support system of FIG.25 according to an embodiment of the disclosure;

FIG. 27 is a lower perspective view of a reinforcement cap according toan embodiment of the disclosure;

FIG. 28 is a sectional view of the reinforcement cap of FIG. 27according to an embodiment of the disclosure;

FIG. 29 is a sectional view of a reinforcement cap having a tappedcenter hole according to an embodiment of the disclosure;

FIG. 30 is a lower perspective view of a reinforcement cap according toan embodiment of the disclosure;

FIG. 31 is a sectional view of the reinforcement cap of FIG. 30according to an embodiment of the disclosure;

FIG. 32 is an upper perspective view of a reinforcement cap with flatsaccording to an embodiment of the disclosure;

FIG. 33 is a sectional view of a conventional rod receptacle inassembly;

FIG. 33A is an enlarged, partial view of FIG. 33;

FIG. 34 is a sectional view of a rod receptacle in assembly havingthreads that define a canted cantilever profile according to anembodiment of the disclosure;

FIG. 34A is an enlarged, partial view of FIG. 34 according to anembodiment of the disclosure;

FIG. 35 is a partial sectional view of a rod receptacle in assemblyhaving reversed canted cantilever threads according to an embodiment ofthe disclosure; and

FIG. 35A is an enlarged, partial view of FIG. 35 according to anembodiment of the disclosure.

DETAILED DESCRIPTION OF THE FIGURES

Referring to FIGS. 1 through 5, an extensible spinal support system 20is depicted according to an embodiment of the disclosure. The extensiblespinal support system 20 as depicted includes an extension ready baseassembly 21 including a pedicle screw 22, a first or base rod receptacle24, a first or base set screw 42, and a first spinal support rod 36(FIG. 1). The extensible spinal support system 20 further includes anextension subassembly 26 and a cap 28 (FIG. 2). The extensionsubassembly 26 includes a base portion 32 and a second or extension rodreceptacle 34. The base rod receptacle 24 is configured to receive thefirst or base spinal support rod 36, and the extension rod receptacle 34is configured to receive a second or extension spinal support rod 38. Inthe depicted embodiments, the base spinal support rod 36 is retainedwithin the base rod receptacle 24 with the base set screw 42. Likewise,for the extensible spinal support system 20, the extension spinalsupport rod 38 is retained within the extension rod receptacle 34 with asecond or extension set screw 44.

The pedicle screw 22 includes a head portion 64 and a threaded shaftportion 65 centered about a central axis 67. The pedicle screw 22extends from a distal end 62 of the base rod receptacle 24, the headportion 64 being captured within the distal end 62 of the base rodreceptacle 24. In some embodiments, a lock ring 66 is captured betweenthe base spinal support rod 36 and the head portion 64 of the pediclescrew 22. In the depicted embodiment, the head portion 64 of the pediclescrew 22 defines a spherical surface portion 68, with the base rodreceptacle 24 including a complementary mating surface 72 that engagesand conforms to the spherical surface portion 68 of the head portion 64of the pedicle screw 22.

Functionally, the spherical surface portion 68 of the head portion 64 ofthe pedicle screw 22 and the complementary mating surface 72 of the baserod receptacle 24 enable the base rod receptacle 24 to pitch about thehead portion 64 of the pedicle screw 22 relative to the central axis 67,and to rotate about the head portion 64 in the pitched orientations. Bythis arrangement, the pedicle screw 22 and the base rod receptacle 24are configured for polyaxial rotation of the base rod receptacle 24about the head portion 64 of the pedicle screw 22.

Herein, “proximal” refers to a direction 76 that is toward a surgeonduring operation or implantation and away from a bone or patient.“Distal” refers to a direction 74 that is away from the surgeon duringoperation or implantation and toward the bone or patient to which theextensible spinal support system 20 is implanted (i.e., a directionopposite the proximal direction 76).

The base rod receptacle 24 includes a side wall 82 having an interiorsurface 84 and an exterior surface 86 (FIG. 1A). A pair of diametricallyoppose slots 88 are defined on the side wall 82, the slots 88 extendingaxially along the side wall 82 and being open at a proximal end 92 ofthe base rod receptacle 24. By formation of the diametrically opposedslots 88, the side wall 82 defines diametrically opposed wall segments90 on opposing sides of the diametrically opposed slots 88. The interiorsurface 84 extends axially along the side wall 82 and includes interiorthreads 94 formed thereon. The exterior surface 86 also extends axiallyand includes exterior threads 96 formed thereon. The base portion 32 ofthe extension subassembly 26 includes a mounting platform 122 and askirt portion 124 that extends from the mounting platform 122 in thedistal direction 74. The skirt portion 124 includes an interior surface123 and an exterior surface 125.

The extension rod receptacle 34 includes a side wall 142 having aninterior surface 144 and an exterior surface 146. A pair ofdiametrically opposed slots 148 are defined on the side wall 142, theslots 148 extending axially along the side wall 142 and being open at aproximal end 152 of the extension rod receptacle 34. By formation of thediametrically opposed slots 148, the side wall 142 defines diametricallyopposed wall segments 150 on opposing sides of the diametrically opposedslots 148. The interior surface 144 extends axially along the side wall142 and includes interior threads 154 formed thereon (FIG. 5). Theexterior surface 146 also extends axially and includes exterior threads156 formed thereon.

In the depicted embodiment, the extension subassembly 26 includes apivot member 170 that attaches the extension rod receptacle 34 to thebase portion 32, the pivot member 170 including a head portion 172 and ashaft portion 174 and defining a through passage 175. In assembly, theshaft portion 174 is concentric with a pivot axis 182 about which theextension rod receptacle 34 is rotatable. The extension rod receptacle34 defines an opening 184 at a distal end 186 thereof. The opening 184is sized to accommodate passage of the shaft portion 174 of the pivotmember 170.

In the depicted embodiment, the mounting platform 122 of the baseportion 32 defines a center hole 188 for receiving the pivot member 170.The extension rod receptacle 34 may include an internal flange 194having an interior face 196, the head portion 172 of the pivot member170 being dimensioned to extend radially over the interior face 196 ofthe internal flange 194 to loosely secure the extension rod receptacle34 to the base portion 32. The head portion 172 of the pivot member 170may be one of several head styles available to the artisan, for examplea flat head (depicted), a socket head, a countersink head, or aspherical head. The interior face 196 of the internal flange 194 may beconfigured to conform to the head portion 172. In some embodiments, acollet 198 is disposed interstitially between the extension spinalsupport rod 38 and the head portion 172. The collet 198 may be a discspring (depicted), lock washer, or other that exerts a bias forceparallel to the pivot axis 182 when compressed between the support rod38 and the head portion 172. The cap 28 includes a top portion 222 and askirt portion 224 that extends from the mounting platform 222. The skirtportion 224 includes interior threads 226 for mating with the exteriorthreads 156 of the extension rod receptacle 34.

In operation, the pedicle screw 22 is inserted into the base rodreceptacle 24 so that the head portion 64 of the pedicle screw 22 can beregistered against the mating surface 72 of the base rod receptacle 24.The pedicle screw 22 is set into the bone of a vertebrae. The lock ring66, if utilized, is disposed within the base rod receptacle 24 andarranged for contact with the head portion 64 of the pedicle screw 22.The base rod receptacle 24 is arranged in a desired orientation on thehead portion 64 of the pedicle screw 22 and the base spinal support rod36 disposed in the base rod receptacle 24. The lock ring 66 is therebydisposed between the base spinal support rod 36 and the head portion 64of the pedicle screw 22. The base set screw 42 is threadably engagedwith the interior threads 94 of the side wall 82 and tightened so thatthe base spinal support rod 36 is clamped between the base set screw 42and the lock ring 66 (if utilized) or, alternatively, the head portion64 of the pedicle screw 22. The tightening of the base set screw 42 alsoseats the head portion 64 of the pedicle screw 22 against the matingsurface 72 of the base rod receptacle 24 to secure the base rodreceptacle 24 in the desired orientation relative to the head portion64.

Referring to FIGS. 6 and 7 and again to FIG. 2, extensible spinalsupport systems 20 having alternative embodiments for the base portion32 are presented according to embodiments of the disclosure. In FIGS. 2,6, and 7, the extensible spinal support systems and their associatedbase portions are referred to collectively and generically by referencecharacters 20 and 32, respectively, and specifically by the referencecharacters 20 and 32 followed by a letter suffix (e.g., extensiblespinal support system 20 a and the associated base portion 32 a).Several components and attributes are common to all extensible spinalsupport systems 20, which are indicated with same numbered referencecharacters.

For the extensible spinal support system 20 a (FIG. 2), the base portion32 a and the base set screw 42 are separate components that areinstalled independently of each other. For the extensible spinal supportsystems 20 a and 20 b, the skirt portion 124 includes interior threads126 for mating with the exterior threads 96 of the base rod receptacle24. For the extensible support system 20 b (FIG. 6), the base portion 32b additionally includes a set screw portion 228 that is integraltherewith, the set screw portion 228 having external threads 232 thatmate with the interior threads 94 of the side wall 82 of the base rodreceptacle 24. Accordingly, for the extensible spinal support system 20b, the external threads 232 of the set screw portion 228 and theinterior threads 226 of the skirt portion 124 of the base portion 32 bare threaded simultaneously. For the extensible spinal support system 20c (FIG. 7), the base portion 32 c also includes the set screw portion228 that is integral therewith, and having external threads 232 thatmate with the interior threads 94 of the side wall 82 of the base rodreceptacle 24. However, for the extensible spinal support system 20 c,an inner surface 234 of the skirt portion 124 is smooth (does notinclude interior threads), such that the skirt portion 124 rotates aboutand slides over but does not threadably engage the exterior threads 96of the base rod receptacle 24. Accordingly, the base portion 32 c matesonly with the interior threads 94 of the side wall 82.

By integrating the set screw portion 228 with the mounting platform 122as in subassemblies 26 b and 26 c, and skirt portion 124 the structuralintegrity of the extensible spinal support system 20 is enhanced. Forexample, lateral forces applied to the extensible spinal support system20 will incur greater resistance because the set screw portion 228 isintegrated with (e.g., unitary with) the mounting platform 122,establishing a shear stress at the junction of the integrated set screwportion 228 and the mounting platform 122 that provides additionalresistance to deformation relative to the extension subassemblies 26 ofFIGS. 10 through 15.

Functionally, each of the extensible support systems 20 provide a uniqueadvantage. Extensible spinal support system 20 a enables mounting of theextension assembly 26 without disturbing the base set screw 42,eliminating the need to reset the base spinal support rod 36. Theextensible spinal support system 20 a may find application wheredisturbance of the base spinal support system is not necessary or is illadvised. The extensible spinal support system 20 b provides a dualthreaded arrangement that enhances structural integrity of the extensionassembly 26 to the base rod receptacle 24. Such enhancement of thestructural integrity may be advantageous for high torque and high stressapplications, such as scoliosis correction. Extensible spinal supportsystem 20 c also includes the integrated set screw portion 228 andattendant benefit while the smooth, sliding fit of the skirt portion 124enables easier installation where the dual threaded arrangement of theextensible spinal support system 20 b may be unnecessary or difficult.The sliding fit of the skirt portion 124 for the extensible spinalsupport system 20 c effectively captures the side walls 82 of the baserod receptacle 24 to limit splaying. The extensible spinal supportsystem 20 c can also be configured for retrofitting with tulips ofconventional spinal support systems that are not “extension ready.”

Referring to FIG. 8, an extensible spinal support system 20 d isdepicted according to an embodiment of the disclosure. As depicted theextensible spinal support system 20 d includes many of the samecomponents and attributes as the extensible spinal support system 20 a,which are indicated with same numbered referenced characters. Whilecomponents and attributes of the extensible spinal support system 20 aare presented in the extensible spinal support system 20 d, suchcomponents and attributes are not limiting. That is, the extensiblespinal support system 20 d may implement various aspects of theextensible spinal support systems 20 b and 20 c as well, which one ofskill in the spinal support arts recognizes in view of this disclosure.

For the extensible spinal support system 20 d, the head portion 172 ofthe pivot member 170 d is spherical, and the extension rod receptacle 34is configured as discussed above for the base rod receptacle 24. By thisarrangement, the pivot member 170 and the extension rod receptacle 34are configured for polyaxial rotation of the extension rod receptacle 34about the head portion 172 of the pivot member 170.

Referring to FIG. 9, the skirt portion 124 of the base portion 34 isdepicted with flats 240 according to an embodiment of the disclosure.The flats 240 are parallel to the pivot axis 182 and may be implementedwith any of the depicted or contemplated embodiments. The embodiment ofFIG. 9 depicts a total of six flats 240. By way of non-limiting example,the number of flats 240 may range from four to twelve. In someembodiments, the skirt portion 124 defines two flats 240 that arediametrically opposed. In some embodiments, the skirt portion 124defines four flats 240 that are distributed as two diametrically opposedpairs that are rotationally offset at 90 degrees with respect to eachother.

Referring to FIGS. 10 through 17, assembly of various extensionsubassemblies 26 are depicted according to embodiments of thedisclosure. In FIGS. 10 through 17, the extension subassemblies andtheir associated pivot members are referred to collectively andgenerically by reference characters 26 and 170, respectively, andspecifically by the reference characters 26 and 170 followed by a lettersuffix (e.g., extension subassembly 26 a and the associated pivot member170 a). Several components and attributes are common to all extensionsubassemblies 26, which are indicated with same numbered referencecharacters.

For the extension subassembly 26 a (FIGS. 10 and 11), the pivot member170 a defines exterior shaft threads 176 formed on an exterior surface178 of the shaft portion 174. The center hole 188 of the mountingplatform 122 includes internal threads 192 for mating with the exteriorshaft threads 176. In some embodiments, the pivot member 170 a includesa stop 242, such as a shoulder 244 having a diameter 246 that is greaterthan the diameter of the exterior shaft threads 176. The stop 242cooperates with the head portion 172 to define an axial gap dimension248 that is greater than an axial thickness 250 of the internal flange194.

In assembly, the pivot member 170 a is inserted through the extensionrod receptacle 34 and into the opening 184 so that the shaft portion 174extends from the distal end 186 of the extension rod receptacle 34 andthe head portion 172 of the pivot member 170 a is within the extensionrod receptacle 34. The shaft portion 174 of the pivot member 170 a andthe center hole 188 of the mounting platform 122 of the base portion 32a are then aligned and the exterior shaft threads 176 of the shaftportion 174 threaded into the internal threads 192 of the center hole188. In some embodiments, the pivot member 170 a is threaded into thecenter hole 188 until the stop 242 is firmly seated against the platform122 over the mouth of the center hole 188 to define the axial gapdimension 248. Alternatively, the pivot member 170 a may otherwiseengage the base portion 32 a in a manner that causes the pivot member170 a to stop within the center hole 188; for example, the threads 176may cease at a point on the pivot shaft 174 that provides the desiredaxial gap dimension 248. Having secured the pivot member 170 a to themounting platform 122, the extension rod receptacle 34 is coupled to thebase portion 32.

For the extension subassembly 26 b (FIGS. 12 and 13), the shaft portion174 of the pivot member 170 b is a right cylinder 252, defining a smoothexterior surface 178. The center hole 188 of the mounting platform 122is also right cylindrical, and may be dimensioned to provide a closesliding fit with the shaft portion 174. In some embodiments, the pivotmember 170 b includes a stop (not depicted) akin to pivot member 170 a.

In assembly, the pivot member 170 b is inserted through the extensionrod receptacle 34 and into the opening 184 so that the shaft portion 174extends from the distal end 186 of the extension rod receptacle 34 andthe head portion 172 of the pivot member 170 b is within the extensionrod receptacle 34. The shaft portion 174 of the pivot member 170 b andthe center hole 188 of the mounting platform 122 of the base portion 32are then aligned and the shaft portion 174 positioned within the centerhole 188 to so that the axial gap dimension 248 is defined between thehead portion 172 and the mounting platform 122. In some embodiments, thepivot member 170 a is threaded into the center hole 188 until the stop242 is firmly seated against the platform 122 over the mouth of thecenter hole 188, thereby defining the axial gap dimension 248. In someembodiments, the length of the shaft portion 174 is dimensioned toprovide the desired axial gap dimension 248 when a distal end 253 of theshaft portion 174 is flush with a distal face 256 of the platform 122.

With the pivot member 170 b positioned in the center hole 188 to definethe axial gap dimension 248 greater than the axial thickness 250 of theinternal flange 194, the pivot member 170 b is secured to the platform122. In the depicted embodiment, the distal end 253 of the pivot member170 b is welded to the distal face 256 of the mounting platform 122 toform a weld 254 at the perimeter of the center hole 188. The weld 254may be continuous, a stitch weld, or a tack weld. The welding operationmay be performed with welding techniques available to the artisan,including but not limited to electron beam welding. Alternatively,instead of welding, the pivot member 170 b may be secured by otherbonding or fusion techniques, such as brazing, soldering, or gluing.Upon securing the pivot member 170 b to the base portion 32, theinternal flange 294 of the extension rod receptacle 34, being capturedbetween the head portion 172 and the mounting platform 122, is coupledto the base portion 32.

For the extension subassembly 26 c (FIGS. 14 and 15), the shaft portion174 of the pivot member 170 c is also the right cylinder 252, definingthe smooth exterior surface 178. The center hole 188 of the mountingplatform 122 is also right cylindrical, and may be dimensioned toprovide a close sliding fit with the shaft portion 174. Similar to pivotmembers 170 a and 170 c, the pivot member 170 c may include the stop 242such as the shoulder 244 with diameter 246, the diameter 246 beinggreater than the diameter of the center hole 188. As with thesubassemblies 26 a and 26 b, the stop 242 of the subassembly 26 ccooperates with the head portion 172 to define an axial gap dimension248 that is greater than an axial thickness 250 of the internal flange194. The distal face 256 of the mounting platform 122 may define arecessed lead in 258 that surrounds the center hole 188.

In assembly, the pivot member 170 c is inserted through the extensionrod receptacle 34 and into the opening 184 so that the shaft portion 174extends from the distal end 186 of the extension rod receptacle 34 andthe head portion 172 of the pivot member 170 c is within the extensionrod receptacle 34. The shaft portion 174 of the pivot member 170 c andthe center hole 188 of the mounting platform 122 of the base portion 32are then aligned and the shaft portion 174 positioned within the centerhole 188 so that the axial gap dimension 248 is defined between the headportion 172 and the mounting platform 122. In some embodiments, thepivot member 170 c is inserted into the center hole 188 until the stop242 registers against the platform 122 over the mouth of the center hole188, thereby defining the axial gap dimension 248. In some embodiments,the length of the shaft portion 174 is dimensioned to provide thedesired axial gap dimension 248 when the distal end 253 is flush withthe distal face 256 of the platform 122.

With the pivot member 170 c positioned in the center hole 188 to definethe axial gap dimension 248 greater than the axial thickness 250 of theinternal flange 194, the pivot member 170 c is secured to the platform122 by a swaging process. The swaging process deforms the distal end 253of the shaft portion 174 into the recessed lead in 258. In this way, themounting platform 122 is swaged between the stop 242 and the deformeddistal end 253 of the pivot member 170 c, akin to a rivet. The internalflange 294 is captured between the head portion 172 of the pivot member170 c and the mounting platform 122 of the base portion 32, therebycoupling the extension rod receptacle 34 to the base portion 32.

The depictions of FIGS. 10 through 15 present base portions 32 akin tobase portion 32 a, i.e., without an integral set screw portion. However,the assembly techniques for the subassemblies 26 described above arereadily implemented for base portions 32 that include the set screwportion 228 integral therewith, akin to base portions 32 b and 32 c. Theembodiments of FIGS. 6 and 7 depict the threaded pivot member 170 a incombination with the integral set screw portion 228. The embodiments ofFIGS. 16 and 17 depict the pivot members 170 b and 170 c with longershaft portions 174 having distal ends 253 that reach the distal faces257 of the integral set screw portions 228. The weld 254 (FIG. 16) andthe recessed lead in 258 (FIG. 17) are formed on the distal faces 257 ofthe integral set screw portion 228. Accordingly, the subassemblies 26may be fabricated with any of the base portions 32 described anddepicted herein.

For the various subassemblies 26, because the axial gap dimension 248 isgreater than the axial thickness 250 of the internal flange 194,monoaxial rotation of the extension rod receptacle 34 about the pivotaxis 182 is achieved. That is, the internal flange 194, thougheffectively captured between the head portion 172 of the pivot member170 and the base portion 32 of the extension subassembly 26, can berotated about the pivot axis 182. In the depicted embodiment, movementof the of the extension rod receptacle 34 relative to the base portion32 is effectively limited substantially to rotation about the pivot axis182, i.e., a “monoaxial” rotation.

For the subassemblies 26 of FIGS. 10-15, because of the exterior threads96 on the side wall 82 of the base rod receptacle 24, the extension rodreceptacle 34 can be mounted to the base rod receptacle 24. That is, theextension subassemblies 26 of FIGS. 10-15 can be mounted directly to theexterior threads 96 of the base rod receptacle 24. For the subassemblies26 of FIGS. 16 and 17, the extension rod receptacle 34 can be mounted tothe interior threads 94 of the base rod receptacle 24, without need forexterior threads on the sidewall 82. Accordingly, because of the varioussubassemblies 26, the base rod receptacle 24 is referred to as“extension ready.”

For the pre-assembled extension subassemblies 26 of FIGS. 10-15, thebase portion 32 is aligned with the base rod receptacle 24 and theinterior threads 126 of the skirt portion 124 of the base portion 32threaded over the exterior threads 96 of the base rod receptacle 24. Forthe pre-assembled extension subassemblies 26 of FIGS. 16 and 17, thebase portion 32 is aligned with the base rod receptacle 24 and both theinterior threads 126 of the skirt portion 124 and the exterior threadsexternal threads 232 of the set screw portion 228 are threadably engagewith the exterior threads 96 of the base rod receptacle 24 and theinternal threads 192 of the center hole 188, respectively. For thesubassemblies 26, because the pivot member 170 is in fixed relationshipwith the base portion 32, the base portion 32 may be drawn tight againstthe proximal end 92 of the base rod receptacle 24 by with a driverinserted in the socket of the pivot member 170. The extension rodreceptacle 34 may be rotated to a desired angular orientation about thepivot axis 182. The extension spinal support rod 38 is inserted into theextension rod receptacle 34, extending laterally through thediametrically opposed slots 148. The extension set screw 44 or,alternatively, the set screw portion 228, is threadably engaged with theinterior threads 154 of the side wall 142 and tightened. When the setscrew 44 is tightened against the extension spinal support rod 38, theextension rod receptacle 34 is drawn in the proximal direction (upwardin FIG. 2) so that the internal flange 194 is drawn tight against thehead portion 172, thereby locking the extension rod receptacle 34 inplace and in a fixed rotational orientation relative to the head portion172.

In some embodiments, in the absence of the extension subassembly 26, thecap 28 can be mounted to the exterior threads 96 of the base rodreceptacle 24. Functionally, this arrangement provides support againstoutward lateral deflections (splaying) of the wall segments 90 wouldotherwise be provided by the skirt portion 124 of the mounting platform122 of the extension subassembly 26. Various reinforcement caps 28 thatcan be implemented with either the base rod receptacle 24 or theextension rod receptacle 34 are described below attendant to FIGS.25-32.

Similar to the extensible spinal support system 20 a of FIG. 2, theextensible spinal support systems 20 b and 20 c of FIGS. 6 and 7 may bemounted to a previously implanted extension ready base assembly withoutneed for removing or otherwise releasing the base spinal support rod 36.The base portions 32 b and 32 c of the extension subassemblies 26 b and26 c may be mounted to the extension ready base assembly 21 of FIG. 1instead of the set screw 42, the base portions 32 b and 32 c serving asa cap for securing the base spinal rod 36 to the base rod receptacle 24.The extension rod receptacle 34 can later be mounted directly to thecenter hole 188 of the mounting platform 122 of the base portion 32 bwith the pivot member 170 a, thereby leaving the arrangement of the baserod receptacle 24 the base portions 32 b, 32 c and the base spinalsupport rod 36 intact. Accordingly, the base rod receptacle 24 incombination with the base portions 32 b or 32 c is also referred to as“extension ready.” In such an embodiment, the extension subassemblies 26b and 26 c are not pre-assembled, but instead assembled on the existingbase portion 32 b, 32 c, being built up from the mounting platform 122as described above.

Referring to FIGS. 18 through 18B, the pedicle screw 22 is depicted inisolation in an embodiment of the disclosure. In addition to thecomponents and attributes discussed above, the pedicle screw 22 maydefine a socket 259 in the head portion 64, accessible from a proximalend 260 of the pedicle screw 22. The threaded shaft portion 65 may bedouble threaded as depicted. In some embodiments, a center passage 261extends through the head portion 64 and threaded shaft portion 65. Inthe depicted embodiment, the socket 259 is hexagonal, but othergeometries, such as a square, rectangle, cross, or star pattern may beutilized.

Functionally, the socket 259 accommodates driving of the pedicle screw22 with an appropriate mating wrench (e.g., hexagonal wrench for thedepicted embodiment, or a square bit, rectangular bit, cross (PHILLIPS)bit, or star (TORR®) bit as appropriate). The center passage 261 may besized, for example, to accommodate sliding passage of a KIRSCHNER wireor a larger diameter rod.

Referring to FIGS. 19 and 20A through 20C, a pedicle screw 22 a isdepicted according to an embodiment of the disclosure. The pedicle screw22 a includes some of the same components and attributes as pediclescrew 22, some of which are identified with same-labeled referencecharacters. In some embodiments, the pedicle screw 22 a includes doublethreads 262 a and 262 b, referred to collectively and generically asthreads or double threads 262. The pedicle screw 22 a may define aself-tapping flute 264 at a distal end portion 265. In some embodiments,the threaded portion 65 of the pedicle screw 22 a defines fenestrations271 that are in fluid communication with the center passage 261. Thepedicle screw 22 a may define a depression 268 at a leading end 269 ofeach double thread 262 a and 262 b.

In some embodiments, the threads 262 that are terminated at theself-tapping flute 264 and include radiused or curved terminations 266at the faces 267 of the self-tapping flute 264, thereby defining aconvex profile that extends tangentially (i.e., in the θ-direction ofthe r-θ-z coordinate of FIG. 19) from proximate a root of the threads262 at the faces 267 of the self-tapping flute 264 to a crest of thethreads 262. In some embodiments, the curved terminations 266 defineradii R. By way of non-limiting example, the radii may be within a rangeof: 100 to 400 micrometers inclusive; 100 to 300 micrometers inclusive;150 to 300 micrometers inclusive; or 150 to 250 micrometers inclusive.Herein, a range that is said to be “inclusive” includes the endpointvalues of the range as well as all values therebetween.

In some embodiments, a leading edge 263 of the flute 264 is rounded orotherwise dulled to blunt an otherwise sharp edge. The edges of thethreaded shaft portion 65, such as corners at the crest of the threads262, may be dulled to blunt an otherwise sharp edge. Techniques forproducing the dulled edges include, for example, anodizing, surfacepolishing, or, in contrast, by surface roughening. Polishing techniquesinclude tumbling the pedicle screw 22 in a granular or powder ceramic.Roughening techniques include sandblasting, wire brushing, laser-inducedroughening, and etching. In some embodiments, the roughening techniquesprovide a surface roughness having an arithmetical mean deviation overthe surfaces of the pedicle screw 22 that is in a range of 5 micrometersto 15 micrometers inclusive. The arithmetical mean deviation parameter,commonly referred to as “Ra” in the texturing arts, is described, forexample, at Degarmo, et al., “Materials and Processes in Manufacturing,”(9th ed.), p. 223, John Wiley & Sons (2003), ISBN 0-471-65653-4, thedisclosure of which is hereby incorporated by reference herein exceptfor express definitions contained therein.

Functionally, the curved terminations 266 reduce the sharpness of thethreads 262 at the faces 267 of the self-tapping flute 264. With thecurved terminations 266, the self-tapping flute 264 is still effectivein tapping into bone tissue, but will not tend to cut soft tissue.Rather than cutting soft tissues, the curved terminations 266, as wellas the dulled corners of the threads 262, tend to push soft tissue asideas opposed to slicing or tearing through the soft tissue. Such softtissue includes blood vessels, which will tend to be deflected (insteadof sliced) by the curved terminations 266. The fenestrations 271 promotethe ingrowth of tissue into the pedicle screw 22 a for a more securemooring of the pedicle screw 22 a within the bone over time.

The depression(s) 268 help compensate for the lack of a sharp cuttingedge at the leading end 269 of the threads 262. In some embodiments, onedepression 268 is formed at the leading end of each of the doublethreads 262 a and 262 b (depicted). The depressions 268 reduce thecircumferential area of the pedicle screw 22 a in the direction ofrotation. For a given applied rotational force, the reduction in areaproduces an increase in the pressure applied by the pedicle screw 22 a.The increased pressure augments penetration of the distal end portion265 of the pedicle screw 22 a through tissue without resort to sharpcutting edges, and also provides a relief that enables the tissue toflow over and around the leading end 269 of the thread 262.

Referring to FIGS. 21 through 23, the base or extension set screw 42, 44is depicted in isolation in an embodiment of the disclosure. In thedepicted embodiments, the base and extension set screws 42 and 44 areidentical and are referred to collectively and generically as the “setscrew 42, 44”. The set screw 42, 44 includes exterior threads 270 thatmate with the interior threads 94, 154 of the base or extension rodreceptacle 24, 34. The set screw 42, 44 may define a socket 272,accessible from a proximal end 274 of the set screw 42, 44. In someembodiments, a center passage 276 extends from the socket 272 through adistal end 278 of the set screw 42, 44. In the depicted embodiment, thesocket 272 is hexagonal, but other geometries, such as a square,rectangle, cross, or star pattern may be utilized.

Functionally, the socket 272 accommodates driving of the set screw 42,44 with an appropriate mating wrench (e.g., hexagonal wrench for thedepicted embodiment, or a square bit, rectangular bit, cross (PHILLIPS)bit, or star (TORX®) bit as appropriate). The center passage 276 may besized, for example, to accommodate sliding passage of a KIRSCHNER wireor larger diameter rod.

Referring to FIG. 24, the lock ring 66 is depicted in an embodiment ofthe disclosure. In some embodiments, the lock ring 66 includes a distalface 282 and a proximal face 284 separated by a perimeter portion 286.The distal face 282 may be convex and define a spherical profile 288.The proximal face 284 may include one or more malleable features 292 forengaging the base spinal support rod 36. The distal face 282 terminatesat a distal edge 294 of the perimeter portion 286. In the depictedembodiment, a plurality of relief slots 296 are defined that are open atthe distal edge 294 and extend axially into the perimeter portion 286.In some embodiments, the one or more malleable features 292 areplastically deformable. The one or more malleable features 292 maydefine a raised ridge 298, for example an annular ring as depicted inFIG. 24. In some embodiments, the lock ring 66 includes a radial detent299 that extends radially outward from the perimeter portion 286.

Functionally, the convexity of the distal face 282 that accommodates andcan slide over the spherical profile of the head portion 64 of thepedicle screw 22, thereby enabling the polyaxial movement of the baserod receptacle 24 relative to the head portion 64. The one or moremalleable features 292 conform to the shape of the base spinal supportrod 36 when the set screw 42 is tightened to secure the base spinalsupport rod 36 in place. The conformance of the malleable feature(s) 292acts to grip the base spinal support rod 36, thereby inhibiting the basesupport rod 36 from rotating or sliding within the diametrically opposedslots 88 of the base rod receptacle 24. Upon tightening of the base setscrew 42, the relief slots 296 enable the perimeter portion 286 toconform to the head portion 64 at the distal edge 294 for more effectivegripping of the head portion 64 of the pedicle screw 22. The conformanceof the malleable feature(s) 292 and perimeter portion 286 act to secureand inhibit movement between the head portion 64, the base spinalsupport rod 36, and the base rod receptacle 24. The radial detent 299may interface with internal features 297 (FIG. 3) within the base rodreceptacle 24, such as an internal inset flange (depicted) or optionallya groove (not depicted). The internal features 297 restrains theproximal face 284 from deflecting proximally (upward in FIGS. 2 and 3)when the lock ring 66 is deformed under the clamping force between thebase spinal support rod 36 and the head portion 64, thereby maintaininggripping contact between the head portion 64 and the proximal face 284of the lock ring 66.

Referring to FIGS. 25 and 26, the extension ready base assembly 21 isdepicted with the reinforcement cap 28 coupled to the base rodreceptacle 24. Referring to FIGS. 27 and 28, a reinforcement cap 28 a isdepicted according to an embodiment of the disclosure. Herein, variousreinforcement caps are presented and referred to collectively andgenerically by reference character 28, and specifically by the referencecharacter 28 followed by a letter suffix (e.g., reinforcement cap 28 a).The reinforcement cap 28 a includes a platform portion 322 including adistal face 324 and a proximal face 326 separated by a perimeter portion328 (FIGS. 26, 27, and 28). A skirt portion 332 extends from the distalface 324 of the platform portion 322 portion, the skirt portion 332including an interior surface 334, at least a portion of which includesinterior threads 336 formed thereon. A set screw portion 342 extendsfrom the distal face 324 of the platform portion 322, the set screwportion 342 being surrounded by the skirt portion 332 and including anexterior surface 338 that faces radially outward. At least a portion ofthe exterior surface 338 includes exterior threads 344 formed thereon.The set screw portion 342 defines a rotation axis 346 about which theset screw portion 342 and the skirt portion 332 are concentric. Anannular gap 348 is defined between the exterior threads 344 of the setscrew portion 342 and the interior threads 336 of the skirt portion 332.

The reinforcement cap 28 a may define a socket 272 accessible from theproximal face 326 of the cap 28 a. In the depicted embodiment, thesocket 272 is hexagonal, but other geometries, such as a square,rectangle, octagon, cross, or star pattern may be utilized. In someembodiments, a center passage 276 extends from the socket 272 through adistal end 278 of the set screw portion 342.

Referring to FIG. 29, a reinforcement cap 28 b modified to define atapped center hole 360 is depicted according to an embodiment of thedisclosure. The reinforcement cap 28 b includes many of the samecomponents and attributes as the reinforcement cap 28 a, which areindicated with same-numbered numerical characters. The tapped centerhole 360 is characterized as having an inner wall 362 on which threads364 are formed, and a bottom surface 366. The socket 272 extendsdistally from the bottom surface 366.

Referring to FIGS. 30 and 31, a reinforcement cap 28 c is depictedaccording to an embodiment of the disclosure. The reinforcement cap 28 cincludes many of the same components and attributes as the reinforcementcap 28 a and 28 b, which are indicated with same-numbered referencecharacters. A distinction of the reinforcement cap 28 c is that theskirt portion 332 does not include interior threads, such that the skirtportion 332 slidingly engages and rotates about the exterior threads 96of the base rod receptacle 24 but does not threadably engage theexterior threads 96. Accordingly, the reinforcement cap 28 c threadablyengages only with the interior threads 94 of the side wall 82. In someembodiments, the skirt portion 332 may define an inner diameter 372 thatslides over the exterior threads 96 with a close, sliding fit.

Referring to FIG. 32, the skirt portion 332 of the reinforcement cap 28is depicted with flats 374 according to an embodiment of the disclosure.The flats 374 extend parallel to the rotation axis 346 and may beimplemented with any of the depicted or contemplated reinforcement caps28. The embodiment of FIG. 32 depicts a total of six flats 374 as anon-limiting example. In some embodiments the number of flats 374 is ina range of six to twelve inclusive. In some embodiments, the skirtportion 332 defines two flats 374 that are diametrically opposed. Insome embodiments, the skirt portion 332 defines four flats 374 that aredistributed as two diametrically opposed pairs that are rotationallyoffset at 90 degrees with respect to each other.

Functionally, the interior threads 94 of the base rod receptacle 24 areconfigured to mate with the exterior threads 344 of the set screwportion 342 of the reinforcement cap 28. For the reinforcement caps 28 aand 28 b, the exterior threads 96 of the base rod receptacle 24 areconfigured to mate with the interior threads 336 of the skirt portion332 of the reinforcement cap 28, 28 b. The socket 272 accommodatesdriving of the reinforcement cap 28, 28 b with an appropriate matingwrench (e.g., hexagonal wrench for the depicted embodiment, or a squarebit, rectangular bit, cross (PHILLIPS) bit, or star (TORX®) bit asappropriate). The flats 374, when implemented, provide an alternativeway to apply torsion to the reinforcement cap 28, for example by use ofsocket tool that slides over and engages the flats 374. The flats 374can also be used in so-called rescue situations, providing alternativegripping surfaces for removal of components of the spinal support system20. The center passage 276 may be sized, for example, to accommodatesliding passage of a KIRSCHNER wire or a guide rod.

The unitary or otherwise integral structure of the set screw portion 342with the platform portion 322 and skirt portion 332 provides additionalstructural strength and integrity relative to a separate cap and setscrew arrangement. For example, the spinal support system 20 providesgreater resistance to lateral forces because the set screw portions 342is integrated with the platform portion 322, establishing a shear stressat a junction 368 of the screw portion 342 and the platform portion 322that provides additional resistance to deformation relative to anassembly where the cap and set screw are separate components. The addedstrength and structural integrity provided by the integrated arrangementof the reinforcement cap 28 may be advantageous for high torque and highstress applications, such as scoliosis correction.

Referring to FIGS. 33, 33A, 34, and 34A, threads defining a cantedcantilever profile 450 and the advantage provided over conventionalthreaded arrangements are depicted and described according toembodiments of the disclosure. Conventional threaded arrangements 400,schematically depicted at FIGS. 33 and 33A, may include, for example,exterior threads 402 of a set screw 404 that are engaged with interiorthreads 406 of a wall segment 408 of a spinal rod receptacle 410 (akinto opposed wall segments 90 and 150 of the rod receptacles 24 and 34 ofthe extensible spinal support system 20). Both the set screw 404 and thewall segment 408 are concentric about a central axis 412 that definesthe z-axis of a right-cylindrical coordinate system 424 having an axialcoordinate z and a radial coordinate r. When the set screw 404 istightened in the first direction 416 to set against a spinal support rod415, a clamping force vector FC is generated, for which there is anequal and opposite force vector FC′ in a second direction 418 that isopposite the first direction 416. The force vector FC′ in turn generatesreaction force vectors FR generated at contact interfaces 422 betweenthe proximal faces of the exterior threads 402 of the set screw 404 andthe distal faces of the interior threads 406 of the wall segment 408.The reaction forces FR generate an axial component FRZ and radialcomponent FRR. Because of the standard shape of the threads 402 and 406,the radial components FRR generate a radial outward force FRO, i.e.,away from the central axis 412.

For configurations such as the depicted extensible spinal support system20, the wall segment 408 (e.g., wall segment 90 of the extensible spinalsupport system 20) is, in some embodiments, not supported by anystructure other than resistance to bending at the base of the wallsegment. In such embodiments, the wall segment 408 will tend to causedeflections δo that deflect radially outward in response to the radialoutward force FRO. As the wall segment 408 deflects radially outward,the overlap between the threads 406 and 408 at the interfaces 422 isreduced, thereby weakening the coupling between the set screw 404 andthe wall segment 408. The tighter the draw on the set screw 404, thegreater the radial outward force FRO and the greater the deflection ofthe wall segment 408, further decreasing the overlap at the interfaces422. Accordingly, as the torque requirements of the conventional setscrew 404 are increased, the coupling between the set screw 404 and thewall segment 408 becomes more tenuous. Over time, creep stresses maycause the deflection of the wall segment 408 (splaying) and theattendant decrease in the overlap at the interfaces 422, causing theclamping force FC to reduce. This can cause loosening of the assemblyand slippage of the resident spinal rod within the spinal rod receptacle410. In some instances, torque requirements can cause the set screw 404to slip within the spinal rod receptacle 410 during implantation.

A remedy for the splaying and attendant slippage of the spinal rod is athreaded arrangement utilizing threads having the canted cantileverprofile arrangement 450, as depicted at FIGS. 34 and 34A. The variousthreads 94, 96, 126, 154, 156, and 270 (FIGS. 2, 4, and 5) of theextensible spinal support system 20 may utilize a canted cantileverprofile arrangement. The canted cantilever profile arrangement 450,schematically depicted at FIGS. 34 and 34A, may include, for example,exterior threads 452 of a set screw 454 that are engaged with interiorthreads 456 of a wall segment 458 of a spinal rod receptacle 460 (akinto opposed wall segments 90 and 150 of the rod receptacles 24 and 34 ofthe extensible spinal support system 20). Both the set screw 454 and thewall segment 458 are concentric about a central axis 462 that definesthe z-axis of a right-cylindrical coordinate system 464 having an axialcoordinate z and a radial coordinate r. When the set screw 454 istightened a first direction 466 to set against a spinal support rod 465,the clamping force vector FC is generated, for which there is the equaland opposite force vector FC′ in a second direction 468 that is oppositethe first direction 466. The force vector FC′ in turn generates reactionforce vectors FR generated at contact interfaces 472 between theproximal faces of the exterior threads 452 of the set screw 454 and thedistal faces of the interior threads 456 of the wall segment 458. Thereaction forces FR generate an axial component FRZ and radial componentFRR.

However, unlike the conventional threaded arrangements 400, the contactinterfaces 472 of the canted cantilever profiles 450 are sloped radiallyinward (i.e., toward the central axis 462) in the first direction 466.By this arrangement, the radial component FRR is vectored inward, towardthe central axis 462. The forces so generated will tend to causedeflections δi of the wall segment 458 that is radially inward inresponse to the radial inward force FRI. Because of the radial inwarddeflections δi, the wall segments 458 tend to be supported by the setscrew 454. Accordingly, the coupling between the set screw 454 and thespinal rod receptacle 460 provided by the canted cantilever profilearrangement 450 is stronger and can provide a greater clamping force FCthan can the conventional threaded arrangement 400 of spinal rodreceptacle 410.

For the extensible spinal support system 20, the interior threads 94,154 of the base and extension rod receptacle 24, 34 interact with theset screws 42, 44 in the manner described attendant to the cantedcantilever profile arrangement 450 of FIGS. 34 and 34A. The exteriorthreads 96, 156 of the base and extension rod receptacle 24, 34 may alsoimplement a canted cantilever arrangement (see, e.g., FIGS. 2, 4, and5), but may be configured to generate different forces and deflections.For example, the exterior threads 96, 156 of the base and extension rodreceptacle 24, 34 are sloped radially outward (i.e., away from thecentral axis 462) in the distal direction 74. By this arrangement, theradial components of the reaction forces at the interface of theexterior threads 96, 156 and the interior threads 126, 226 of the skirtportions 124, 224 is vectored outward, away from the center axis 67. Theforces so generated will tend to cause the wall segments 90, 150 todeflect radially outward. Because of the radial outward deflections, thewall segments 90, 150 tend to be supported by the skirt portions 124,224. The skirt portions 124, 224, being tangentially continuous, incursa hoop stress that further counters the outward radial forces and limitssplaying. The outward radial forces at the exterior threads 96, 156 alsotend to counter and can be tailored to balance the inward radial forcesFRI to further reduce overall radial deflection and deformation of thewall segments 90, 150.

Alternatively, the exterior threads 96, 156 of the receptacles 24, 34and the interior threads 126, 226 of the skirt portions 124, 224 may beof a conventional arrangement. Conventional threads, as describedattendant to FIGS. 33 and 33A, provide radial outward forces that aresubsequently supported by the skirt portions 124, 224 and as a counterto the radial inward force FRI, to prevent splaying.

Referring to FIGS. 35 and 35A, a partial view of the extensible spinalsupport system 20 having a reinforcement cap 28 d and rod receptacle 24,34 that define a reverse canted cantilever profile 480 is depictedaccording to an embodiment of the disclosure. The reinforcement cap 28 dincludes many of the same components and attributes as the reinforcementcaps 28 a and 28 b, as well as the reaction vectors of FIGS. 34 and 34A,all of which are indicated with same-numbered reference characters. Adistinction of the FIGS. 35 and 35A embodiment is that the exteriorthreads 96, 156 of the receptacles 24, 34 are sloped radially outward(i.e., away from the rotation axis 346) in the proximal direction 76instead of the distal direction 74, with the interior threads 336 of theskirt portion 332 of the reinforcement cap 28 d being configured to matetherewith.

In FIG. 35A, the reaction force vector FR at the contact interface 472and the components FRR and FRZ thereof are superimposed from FIG. 34A atthe exterior threads 344 of the set screw portion 342 and the interiorthreads 94, 154 of the base or extension rod receptacle 24, 34. Areaction force vector FR′ at a contact interface 472′ and the componentsFRR′ and FRZ′ thereof are depicted at the interior threads 336 of theskirt portion 332 and the exterior threads 96, 156 of the base orextension rod receptacle 24, 34.

Functionally, by this arrangement, not only do the reaction forcevectors FR at the contact interface 472 generate the radial componentFRR vectored inward (i.e., toward the center axis 466), but so do thereaction force vectors FR′ at the contact interfaces 472′. Accordingly,both the contact interfaces 472 and 472′ contribute to the radial inwardforce FRI that opposes the splaying that may result from the setting ofthe set screw portion 342.

The depictions of FIGS. 35 and 35A illustrate the reinforcement cap 28 din assembly on the base or extension rod receptacle 24, 34. The samearrangement for the reverse canted cantilever profile 480 may beemployed mutatis mutandis with the various base portions 32 in assemblywith the rod receptacle 24, which one of ordinary skill in the pediclescrew and tulip arts understands in view of this disclosure.

Each of the additional figures and methods disclosed herein can be usedseparately, or in conjunction with other features and methods, toprovide improved devices and methods for making and using the same.Therefore, combinations of features and methods disclosed herein may notbe necessary to practice the disclosure in its broadest sense and areinstead disclosed merely to particularly describe representative andpreferred embodiments.

Various modifications to the embodiments may be apparent to one of skillin the art upon reading this disclosure. For example, persons ofordinary skill in the relevant arts will recognize that the variousfeatures described for the different embodiments can be suitablycombined, un-combined, and re-combined with other features, alone, or indifferent combinations. Likewise, the various features described aboveshould all be regarded as example embodiments, rather than limitationsto the scope or spirit of the disclosure.

Persons of ordinary skill in the relevant arts will recognize thatvarious embodiments can comprise fewer features than illustrated in anyindividual embodiment described above. The embodiments described hereinare not meant to be an exhaustive presentation of the ways in which thevarious features may be combined. Accordingly, the embodiments are notmutually exclusive combinations of features; rather, the claims cancomprise a combination of different individual features selected fromdifferent individual embodiments, as understood by persons of ordinaryskill in the art.

Any incorporation by reference of documents above is limited such thatno subject matter is incorporated that is contrary to the explicitdisclosure herein. Any incorporation by reference of documents above isfurther limited such that no claims included in the documents areincorporated by reference herein. Any incorporation by reference ofdocuments above is yet further limited such that any definitionsprovided in the documents are not incorporated by reference hereinunless expressly included herein.

Unless indicated otherwise, references to “embodiment(s)”, “disclosure”,“present disclosure”, “embodiment(s) of the disclosure”, “disclosedembodiment(s)”, and the like contained herein refer to the specification(text, including the claims, and figures) of this patent applicationthat are not admitted prior art.

For purposes of interpreting the claims, it is expressly intended thatthe provisions of 35 U.S.C. 112(f) are not to be invoked unless thespecific terms “means for” or “step for” are recited in the respectiveclaim.

What is claimed is:
 1. A vascular-safe pedicle screw, comprising: a headportion; a threaded shaft portion distal to said head portion andincluding threads that extend to a distal end portion of said threadedshaft portion; and a self-tapping flute at the distal end portion ofsaid threaded shaft portion, said self-tapping flute being recessed intosaid threaded shaft to define at least one terminated thread at a faceof said self-tapping flute, wherein said at least one terminated threadis curved to define a convex profile that extends in a tangentialdirection from proximate a root of said at least one terminated threadat said face of said self-tapping flute to a crest of said at least oneterminated thread.
 2. The vascular-safe pedicle screw of claim 1,wherein said convex profile defines a radius.
 3. The vascular-safepedicle screw of claim 1, wherein said threads are double threads. 4.The vascular-safe pedicle screw of claim 1, wherein said head portionand said threaded shaft portion define a central passage that passestherethrough.
 5. The vascular-safe pedicle screw of claim 4, whereinsaid threaded portion defines a plurality of fenestrations that are influid communication with said central passage.
 6. The vascular-safepedicle screw of claim 1, wherein said distal end portion of saidthreaded shaft portion defines a depression at a leading end of saidthreads.
 7. The vascular-safe pedicle screw of claim 1, wherein saidthreaded shaft portion includes dulled edges.
 8. The vascular-safepedicle screw of claim 7, wherein said dulled edges are polished.
 9. Thevascular-safe pedicle screw of claim 7, wherein said dulled edges areroughened.
 10. The vascular-safe pedicle screw of claim 9, wherein saidthreaded shaft portion defines a surface roughness having arithmeticalmean deviation that is in a range of 5 micrometers to 15 micrometersinclusive.
 11. A spinal rod support system, comprising: a pedicle screw,including: a head portion; a threaded shaft portion distal to said headportion and including threads that extend to a distal end portion ofsaid threaded shaft portion; and a self-tapping flute at the distal endportion of said threaded shaft portion, said self-tapping flute beingrecessed into said threaded shaft to define terminations of said threadsat a face of said self-tapping flute, said terminations being defining aconvex profile that extends in a tangential direction from a root to acrest of said threads at said face of said self-tapping flute; a baserod receptacle coupled to said pedicle screw; and an extension rodreceptacle configured to couple to said base rod receptacle.
 12. Thespinal rod support system of claim 11, wherein said threaded shaftportion includes dulled edges.
 13. The spinal rod support system ofclaim 11, comprising a base portion configured to threadably couple tosaid base rod receptacle, the base portion including a skirt portionthat extends from a mounting platform, said mounting platform defining acenter hole accessible from a proximal face of said mounting platform,said extension rod receptacle extending into said center hole.
 14. Thespinal rod support system of claim 13, wherein: said base rod receptacledefines exterior threads; said skirt portion of said base definesinterior threads configured to mate with said exterior threads of saidbase rod receptacle.
 15. The spinal rod support system of claim 14,comprising a reinforcement cap configured to couple to one of said baserod receptacle and said extension rod receptacle, said reinforcement capincluding a set screw portion and a skirt portion, wherein: said baserod receptacle defines interior threads; said extension rod receptacledefines interior threads; and said set screw portion of saidreinforcement cap defines exterior threads configured to mate witheither of said interior threads of said base rod receptacle or saidinterior threads of said extension rod receptacle.
 16. The spinal rodsupport system of claim 15, wherein: said extension rod receptacledefines exterior threads; said skirt portion of said reinforcement capdefines interior threads configured to mate with either of said exteriorthreads of said base rod receptacle or said exterior threads of saidextension rod receptacle.
 17. The spinal rod support system of claim 16,wherein: said interior threads of said base rod receptacle and saidinterior threads of said extension rod receptacle each define a firstcanted cantilever profile that extends radially inward and in a distaldirection; and said exterior threads of said base rod receptacle andsaid exterior threads of said extension rod receptacle each define asecond canted cantilever profile that extends radially outward and in aproximal direction.